Proton Precipitation During Detached Subauroral Arc Events:
Observations and Modeling

V.K. Jordanova


Detached proton auroral arcs have been recently observed at Earth by the
instruments on IMAGE satellite. These events are characterized by IMAGE Far
Ultraviolet (FUV) observations of subauroral arcs, at lower latitudes and
separated from the main auroral oval, and extending over several hours of local
time in the afternoon sector. The emissions have been related to the
precipitation of 20-30 keV protons measured by the FAST electrostatic analyzers.
Plasmaspheric plumes were simultaneously observed during many of these events by
the Extreme Ultraviolet (EUV) instrument on IMAGE and the Magnetospheric Plasma
Analyzer (MPA) on the geosynchronous Los Alamos spacecraft. In this study we
present simulations of the proton precipitation during several subauroral arc
events identified from IMAGE data sets. The inner magnetospheric conditions were
moderately disturbed during these periods. We employ our global physics-based
model, which calculates the evolution of ring current H+, O+, and He+ ion
distributions due to time-dependent earthward transport and acceleration
[Jordanova et al., J. Geophys. Res., 7, 2001]. We use a high resolution
convection model, where the strength of the cross-tail electric field is
inferred from the latitude of the equatorward edge of the diffuse aurora. The
ring current model is self-consistently coupled with a time-dependent
plasmasphere model through the employed electric and magnetic fields. All major
loss processes are included in our kinetic model, namely, charge exchange,
Coulomb collisions, wave-particle interactions, loss due to collisions with the
dense atmosphere, and convective loss through the dayside magnetopause.
Measurements from the geosynchronous LANL satellites are used to simulate the
time-variant plasma inflow on the nightside and to compare with model results on
the dayside. The growth rate of electromagnetic ion cyclotron (EMIC) waves is
self-consistently calculated as the time progresses and global images of
precipitating ions are obtained. In good agreement with IMAGE observations, EMIC
waves are preferentially excited, and proton precipitation maximizes, within
regions of spatial overlap of energetic ring current protons and cold
plasmaspheric populations in the afternoon sector. This indicates that pitch
angle scattering by EMIC waves is a viable mechanism for particle precipitation
into the atmosphere and generation of detached proton auroral arcs.

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Presentation, 2nd Annual Asia Oceanic Geosciences Society Meeting, Singapore 
20-24 June 2005